This invention generally relates to a process and apparatus, and more specifically, the present invention is directed to a process and apparatus for preparing large area thin film multilayers for thin film transistors, which can be used, for example, as electrical current switches in liquid crystal displays; in optical sensor arrays, and in multiplexed output devices. Large area thin film fabrication is accomplished in accordance with the present invention by effecting the deposition of layered amorphous hydrogenated silicon compositions on a polygon electrode situated in vacuum chamber, wherein appropriate gas source materials are subjected to decomposition in an electrical discharge subsequent to causing these gases to flow toward the polygon, preferably in a crossward direction thereof, or in a direction orthogonal to the substrate axis.
There is described in a copending application, U.S. Ser. No. 456,935/83, the disclosure of which is totally incorporated herein by reference, an apparatus for the preparation of semiconducting and photoelectric devices, inclusive of amorphous hydrogenated silicon imaging members. In one embodiment of the invention described in the copending application, there is illustrated an apparatus for preparing amorphous hydrogenated silicon photoconductive devices comprised in operative relationship of a first electrode substrate means, containing heating elements therein, a counterelectrode means, a receptacle, or vacuum chamber, for housing the first electrode means and the counterelectrode means, with the first electrode means containing the substrate or workpiece, which may be in the form of cylindrical member or drum, an entrance means and an exhaust means in the receptacle for a silane gas. In a variation of this apparatus, a plurality of modules consisting of a first electrode means with heating elements therein, a second counterelectrode means and substrates on the first electrode means, can be arranged in a single receptacle or chamber which contains one gas inlet and one gas exhaust exit. The process and apparatus of the present invention, while similar to that described in the copending application, differs therefrom with respect to the selection of a polygon electrode, which can be used as anode or cathode during the excecution of the process; and the use of a discharge shield for anodic operation.
There is also disclosed in a copending application, U.S. Ser. No. 548,117/83, layered photoresponsive devices comprised of amorphous hydrogenated silicon as a charge carrier transport layer, situated between a supporting substrate, and a thin trapping layer of heavily doped amorphous hydrogenated silicon and overcoating layers of, for example, silicon nitride, silicon carbide, amorphous carbon, and the like on top of the trapping layer. In a specific embodiment, described in this copending application, there is illustrated a photoresponsive device comprised in the order stated of (1) a supporting substrate, (2) a carrier transport layer comprised of uncompensated or undoped amorphous hydrogenated silicon, or amorphous hydrogenated silicon slightly doped with p or n type dopants such as boron or phosphorous, (3) a trapping layer comprised of amorphous hydrogenated silicon which is heavily doped with p or n type dopants such as boron or phosphorous, and (4) a top overcoating layer of silicon nitride, silicon carbide, or amorphous carbon, wherein the top overcoating layer can be optionally rendered partially conductive as illustrated hereinafter. The imaging member of this copending application, the disclosure of which is totally incorporated herein by reference, can be prepared as described in copending application U.S. Ser. No. 456,935.
Additionally, there is described in the prior art methods for obtaining amorphous hydrogenated silicon by glow discharge processes. In this process, the vapor deposition of a silane gas occurs by causing the gas to decompose between two electrodes, one of which has a substrate contained thereon. As electrical power is applied to the electrodes, the silane gas decomposes into reactive silicon-hydrogen species, which will deposit as a solid film on both electrodes. The presence of hydrogen can be of critical importance since it tends to partially coordinate with the dangling bonds in the silicon, as the mono, di, and tri-hydrides, thereby serving to passivate these bonds.
In another known process, amorphous hydrogenated silicon can be prepared by a sputtering technique, wherein a substrate is attached to one electrode, and a target of silicon is placed on a second electrode. These electrodes are connected to a high voltage power supply and a gas, which is usually a mixture of argon and hydrogen, is introduced between the electrodes to provide a medium in which a glow discharge, or plasma can be initiated, and maintained. The glow discharge provides ions which strike the silicon target, and cause the removal by momentum transfer of mainly neutral target atoms. These atoms subsequently condense as a thin film on the substrate electrode. Also, the glow discharge functions to activate the hydrogen, causing it to react with the silicon, and be incorporated into the deposited silicon film. The activated hydrogen also coordinates with the dangling bonds of the silicon to form mono, di, and tri-hydrides.
There is also known, an apparatus and process for preparing amorphous hydrogenated silicon films on a substrate, which involves means for directing and accelerating an ion beam from a plasma toward a sputtering target contained within a chamber, which chamber also contains a shield means having a low sputtering efficiency compared to the sputtering target. The shield means is situated between stray ion beams and the vacuum chamber surface. More specifically, this ion beam process involves producing semiconductive films on a substrate comprising generating a plasma, directing and accelerating an ion beam of the plasma toward a sputtering target, contained in a vacuum chamber at reduced pressure, shielding the vacuum chamber surface from stray ion beams, whereby sputtering of the vacuum chamber surface by the plasma is minimized; followed by sputtering the target with the ion beam, and collecting the sputtered target material as a film on the substrate. The substrate is spatially separated from the plasma generating process and the sputtering process.
Additionally, there is disclosed in U.S. Pat. No. 4,265,991 an amorphous hydrogenated silicon photoconductor. This patent describes several processes for preparing amorphous hydrogenated silicon. In one process, there is prepared an electrophotographic photosensitive member which involves heating the electrophotographic member contained in a chamber to a temperature of 50.degree. C. to 350.degree. C., introducing a gas containing hydrogen into the deposition chamber, causing an electrical discharge in the space of the deposition chamber, in which a silicon compound is present, by electric energy to ionize the gas, followed by depositing amorphous hydrogenated silicon on the electrophotographic substrate at a rate of 0.5 to 100 Angstroms per second by utilizing an electric discharge, while increasing the temperature of the substrate, thereby resulting in an amorphous hydrogenated silicon photoconductive layer of a predetermined thickness.
Moreover, several methods have been described for the plasma deposition of various substances, inclusive of amorphous hydrogenated silicon and silicon nitride on flat substrates. In many of these methods the substrates are maintained in a stationary position with respect to the gas flow and the counterelectrode. In some of these processes gas depletion effects are substantially avoided by using a tapered or a central exhaust, reference the known Reinberg reactor. While this type of reactor is extensively used for etching and deposition processes, its use thereof for scaleup purposes is not practical in view of the gas depletion effects which change non-linearly with gas flowrate. With the apparatus of the present invention the deposition geometry is desirably linearly scaleable to arbitrarily large throuphputs and enables the yielding of uniform film thicknesses on large substrates. Thus the geometry of the apparatus of the present invention is linearly scaleable in the direction of the axis of the polygon electrode, and by the number of polygon electrodes, with associated counterelectrodes contained in the same vacuum system.
Other types of large area flat plate reactors select sample trays which are linearly moved through a reaction zone. These reactors suffer from the disadvantage of mechanical complexity. Therefore, in these reactors the mechanical transmission mechanism that moves the substrate tray is inevitably undesirably covered with the material which is deposited in the reaction zone. The mechanical motion involved causes some fraction of the deposited material to spall off during the process. It is these loose particles which often result in coating defects, and reduce the process yield. In yet another type of reactor, a long sheet of substrate material is transferred from roll to roll through the reaction zone. Although this reactor may not suffer from the aformentioned flake problem, its application is limited to the use of flexible substrates.